This invention pertains generally to a system for measuring the rate of flow of coolant within a nuclear reactor coolant loop and more particularly to such a system which utilizes the statistical variation of the N-16 activity within the coolant to obtain the flow rate measurement.
The N-16 transit time flowmeter of U.S. Pat. No. 3,818,231 utilizes the cross-correlation of the output signals generated from two N-16 gamma detectors positioned at spaced locations along a reactor coolant pipe to determine the transit time of corresponding N-16 perturbations traversing the pipe between detector locations; and thus establishes the corresponding coolant flow rate. In order that systematic errors are not introduced by the electronics, which process each of the detector outputs, the electronics channels for each detector must be extremely and accurately matched. Each detector channel generally includes a picoammeter front end to convert the very low detector current signal of approximately 3.times.10.sup.-8 amperes to voltage signals, high and low pass filters to reject the DC component and high frequency components, and an analog-to-digital converter to digitize the signals which are then cross-correlated in a digital cross-correlator. The degree of matching required to avoid phase shift errors in the cross-correlation function is difficult to obtain and maintain in a plant environment. Without obtaining this degree of matching, any phase shift present is likely to add an error to the measured transit time between N-16 detectors.
Accordingly, a new system is desired that will eliminate systematic errors due to imperfect phase shift matching between the two analog input channels. Preferably, such a system should identify any excessive drift in the electronics and avoid the necessity for on-line calibrations. If the source of error is eliminated, the degree of matching required for the electronics could be significantly reduced, reducing the cost of the electronics, while increasing the accuracy and reliability of the overall system.